You misunderstand how trespassing works here. Civil law, as used in Belgium and most of the world, is completely different from Common law used in English-speaking countries.

Trespassing (lokaalvredebreuk or huisvredebreuk) has a much narrower definition focused on squatting for the former, or entering a home for the latter. A fenced-off party area in a public space is neither. Even if it were trespassing, police can't just force people to leave on the spot because someone asked them to.

The whole issue is that the lawful basis for ejecting the journalists is very unclear, and the initial complaint (active threat) certainly wouldn't play in Freedom 250's favor if it reached a court.